net: mdio-mux-mmioreg: Can handle 8/16/32 bits registers

This patch fix an old information that mdio-mux-mmioreg can only handle
8bit registers.
This is not true anymore.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>